Characterization of a half-wave plate for cosmic microwave background circular polarization measurement with POLARBEAR.

This article focuses on the characterization of the half-wave plate (HWP) used in the Polarbear cosmic microwave background (CMB) experiment, specifically evaluating its leakage between circular polarization (CP) and linear polarization signals. Using laboratory transmission measurements of linearly polarized light across 120–160 GHz and a physical model fitted via Markov Chain Monte Carlo methods, the study precisely determines the HWP's optical parameters and estimates the band-averaged CP-to-linear polarization leakage coefficient. Results show a nonzero leakage coefficient, with values ranging approximately from 0.002 to 0.133 depending on the detector wafer and assumed CP source spectrum, indicating that Polarbear has sensitivity to CP signals. The paper also assesses systematic uncertainties related to detector bandpass and incident light angle, finding them smaller than statistical errors, and discusses prospects for using Polarbear data to probe CP anisotropies, including potential cross-checks via atmospheric Zeeman emission.